β-amylase genes in common/bread wheat (Triticum aestivum)

Abstract

β-amylase is an exoenzyme that releases successive maltose units from the non-reducing end of a polysaccharide chain. This thesis examined the complement of β-amylases in wheat, placing focus on the endosperm-specific (BAM1) and tissue-ubiquitous (BAM2) β-amylases. Ten forms of β-amylase genes were identified in bread wheat. Nine of these had homologues in other cereals, but only BAM1 forms had homologues in Triticeae species. The BAM1 gene sequences showed that there were three types of Gly-rich repeat sequences containing putative N-myristoylation sites: a five-repeat form and two forms with three repeats. There were three main forms of BAM1 genes in wheat cultivar Chinese Spring (CS) of 525, 526 and 550 amino acids in length, respectively. Each form was found to be unique to one chromosome per genome. Each form had within-form variation, and only one copy of each form was found to be an active gene. PCR performed on CS nullisomic-tetrasomic lines showed that there were at least five, three and four copies of BAM1 gene on chromosomes 4B, 4D and 5A, respectively. Exons II, V and VI of BAM1 genes were the most variable regions, enriched with numerous putative single-nucleotide-polymorphic sites. PCR showed that the most variable regions were introns II and III, which were variable both between copies of the same cultivar and between cultivars. This research established that BAM1 genes had undergone limited variation, especially along the coding sequence, during evolution, as well as after polyploidisation. This was supported by genomic and EST sequences of BAM1 that are 99-100% identical in diploid, tetraploid and hexaploid wheat and similar electrophoretogram patterns of PCR amplicons. In conclusion, wheat β-amylases are a complex group of genes found on 28 loci in the hexaploid wheat genome. Further research is needed to understand their functions and their potential utilisation in breeding and industry.